Ink composition, ink set using same, recording method, ink head and recorded image

ABSTRACT

An object of the present invention is to provide an ink composition having a high permeability as an inkjet ink and permitting high quality image printing even with a high speed. 
     To achieve the above object, the present invention provides an ink composition including:
     (a) an aqueous medium having a conductivity of 250 μS/cm (at 25° C.) or lower;   (b) a pigment; and   (c) a compound represented by the formula (I):   

                         
wherein m and n each are an integer of 0-11; m+n is an integer of 9-11; x is an integer of 5-9; and y is a number between 2.5-5.

TECHNICAL FIELD

The present invention relates to an ink composition. More particularly,the present invention relates to the ink composition suitably used in aninkjet recording method, and an ink set, a recording method, an ink headand a recorded image using the ink composition.

BACKGROUND ART

Inkjet recording is a typical non-impact recording method to produceimages by letting ink drops fly onto a medium in response to digitalsignals, which is a method by forming ink drops by means of mechanicalor thermal action and producing images on an image-recording elementsuch as paper by using the ink drops.

The inkjet recording does not involve a process such as development orfixing needed in recording methods such as electrophotography and thuspermits easy device downscaling as well as high-speed and low-noiserecording.

Owing to relative inexpensiveness of recording devices and easiness ofcolorization, this recording method has been widely used in recent yearswith output devices for office machines and for consumer-orientedpersonal computers.

With this being the situation, it is most desired for this recordingmethod to be able to print at a high speed and stably images having highquality comparable to that of electrophotographs.

Printing at a high speed and stably needs accelerating ink drying on amedium. A method of accelerating drying by using a combination ofalcoholic amine and fat dye is disclosed in, for example, JapaneseExamined Patent Publication No. Sho 61(1986)-33870. However, this methodhas a problem in print quality.

On the other hand, in high quality image printing, a method bysimultaneous performance of heating and printing to ensure high qualityimages is disclosed in, for example, Japanese Unexamined PatentPublication Hei 11(1999)-129460. However, in high speed printing by thismethod, there will be unevenness in propagation of heat, resulting in afailure to keep high quality. Furthermore, in such a case, powerconsumption might potentially increase.

Also, there have currently been few discussions concerning the use of apigment as a coloring material.

DISCLOSURE OF INVENTION

An object of the present invention is to provide an ink composition:capable of simultaneously achieving quick drying and high quality imageprinting, contrary goals as described; having a high permeability as aninkjet ink; and permitting high quality image printing even with a highspeed.

According to the present invention, there is provided an ink compositioncomprising:

-   (a) an aqueous medium having a conductivity of 250 μS/cm (at 25° C.)    or lower;-   (b) a pigment; and-   (c) a compound represented by the formula (I):

wherein m and n each are an integer of 0-11; m+n is an integer of 9-11;x is an integer of 5-9; and y is a number between 2.5-5.

Also, according to the present invention, there are provided

(1) an ink set comprising:

the above ink composition, wherein the pigment is C.I. Pigment Blue 15:3or C.I. Pigment Blue 15:4;

the above ink composition, wherein the pigment comprises at least onepigment selected from the group consisting of C.I. Pigment Red 122, C.I.Pigment Red 209 and C.I. Pigment Violet 19; and

the above ink composition, wherein the pigment comprises at least onepigment selected from the group consisting of C.I. Pigment Yellow 74,C.I. Pigment Yellows 128 and 138, and C.I. Pigment Yellow 180, and

(2) an ink set comprising:

the above ink composition, wherein the pigment is C.I. Pigment Blue 15:3or C.I. Pigment Blue 15:4;

the above ink composition, wherein the pigment comprises at least onepigment selected from the group consisting of C.I. Pigment Red 122, C.I.Pigment Red 209 and C.I. Pigment Violet 19;

the above ink composition, wherein the pigment comprises at least onepigment selected from the group consisting of C.I. Pigment Yellow 74,C.I. Pigment Yellows 128 and 138, and C.I. Pigment Yellow 180; and

the above ink composition, wherein the pigment is a carbon black.

Furthermore, according to the present invention, there is provided arecorded image which is recorded by using the above ink composition.

By including the compound of the formula (I), the ink compositionpermits ink ejection stability even in high speed printing and has anexcellent property of quick drying of ink and low-bleeding property,thereby achieving high quality print.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an exploded perspective view showing, in a simplified form,the construction of a piezo-type ink head 1 according to an embodimentof the present invention (driving electrodes of FIG. 2 are omitted);

FIG. 2 is a plan view showing part of the construction of the ink head 1shown in FIG. 1;

FIG. 3 is a cross sectional view of the ink head 1 viewed along thedirection in which the ink chambers 40 extend;

FIG. 4 is an exploded perspective view showing, in a simplified form,the construction of an ink head 2 according to another embodiment of thepresent invention;

FIG. 5 is a plan view showing part of the construction of the ink head 2shown in FIG. 4; and

FIG. 6 is a view showing the state in which a drop 61 of the inkcomposition 60 is ejected from the ink chamber 40.

BEST MODE FOR CARRYING OUT THE INVENTION

An ink composition of the present invention includes a compoundrepresented by the formula (I):

wherein m and n each are an integer of 0-11; m+n is an integer of 9-11;x is an integer of 5-9; and y is a number between 2.5-5. The compound ofthe formula (I) provides an ink with a property hard to foam, and has agood application properties with respect to members of a printer nozzle.As a result, even if foams are generated in the ink, they are relativelyprone to disappear to ensure stable ink feed in high speed printing.Furthermore, the composition has the effect of accelerating thepermeation of the ink into paper in printing on a medium, especially onordinary paper.

The compound of the formula (I), by being contained in a criticalmicelle concentration or above, for example in a concentration of0.1-2.0 parts by weight, more specifically in a concentration of 0.5-1.5parts by weight in the above ink composition, has the effects asdescribed above.

In the ink composition of the present invention, an aqueous medium has aconductivity of 250 μS/cm (at 25° C.) or lower. This is because if theconductivity is higher than 250 μS/cm (at 25° C.), the aqueous mediumcontains impurities in many cases, potentially resulting in a failure toexhibit a desired effect to a full extent.

Herein, the content of the aqueous medium according to the presentinvention is not particularly limited as long as the conductivitythereof is in the above range.

The ink composition of the present invention preferably has aconductivity of 8 mS/cm (at 25° C.) or lower, more specifically between0.05 and 5 mS/cm. Also, the viscosity is preferably 20 mPa·S or lower,more specifically 15 mPa·S or lower in order for drops of the inkcomposition to be stably ejected. The surface tension is preferably20-60 mN/m, more specifically 20-50 mN/m.

An pigment to be used in an amount of 0.1-10 parts by weight in the inkcomposition of the present invention is not particularly limited, butpreferably it is a self-dispersion type pigment obtained by a knownmethod such as introducing a carboxylic acid or a sulfonic acid on itssurface, and more preferably it is a pigment having excellent waterresistance, light resistance and dispersion stability.

Examples of cyan pigments include, for example, C.I. (Color Index)Pigment Blues 1, 2, 15, 15:1, 15:2, 15:3, 15:4, 16, 17, 21, 22, 60, 64and the like. By including as a pigment C.I. Pigment Blue 15:3 or C.I.Pigment Blue 15:4 among these, the ink composition can realize recordedimages having excellent cyan color development properties.

Examples of magenta pigments include, for example, C.I. Pigment Reds 1,2, 3, 5, 10, 16, 23, 30, 31, 48:1, 48:2, 49:1, 52, 53:1, 57:1, 58:4, 63,122 and 209, as well as C.I. Pigment Violet 19 and the like. Byincluding as a pigment at least one pigment selected from the groupconsisting of C.I. Pigment Red 122, C.I. Pigment Red 209 and C.I.Pigment Violet 19 among these, the ink composition can realize recordedimages having excellent magenta color development properties.

Examples of yellow pigments include, for example, C.I. Pigment Yellows3, 12, 13, 14, 15, 23, 55, 74, 83, 93, 95, 97, 128, 138, 150, 154, 167,180, 193 and the like. By including as a pigment at least one pigmentselected from the group consisting of C.I. Pigment Yellow 74, C.I.Pigment Yellows 128 and 138, and C.I. Pigment Yellow 180 among these,the ink composition can realize recorded images having excellent yellowcolor development properties.

Examples of black pigments include, for example, a channel black, afurnace black, a thermal black, a lamp black and a carbon black. Byincluding as a pigment a carbon black among these, the ink compositioncan realize recorded images having excellent black color developmentproperties.

These color pigments can be arbitrarily combined, and for example, anink set including a combination of cyan, magenta and yellow can realizerecorded images in deep black color similar in color concentration torecorded images to be formed from a black color ink composition. Also,an ink set including a combination of cyan, magenta, yellow and blackcan suitably reproduce black color and realize full-color recordedimages having excellent color balance and color development properties.

The ink composition of the present invention preferably comprises atleast one water-soluble organic solvent having a vapor pressure higherthan that of water, and specifically a vapor pressure of 0.05 mmHg orlower at 20° C. and the total amount of organic solvents including otherwater-soluble organic solvents is preferably 10-70 wt % and morepreferably 10-50 wt %. This assists to inhibit the ink composition fromdrying and improve the ejection stability.

Specific examples of water-soluble organic solvents include, forexample, glycol ethers (e.g., ethylene glycol monomethyl ether,diethylene glycol monomethyl ether, ethylene glycol monobutyl ether,diethylene glycol monoethyl ether, triethylene glycol monobutyl ether,propylene glycolmonoethyl ether, tetraethylene glycol monomethyl ether,ethylene glycol monophenyl ether and the like), alcohols (e.g.,methanol, ethanol, propanol, isopropanol, 1,5-pentanediol,1,4-butanediol, 1,3-propanediol and the like), polyhydric alcohols(e.g., 1,2,6-hexanetriol, glycerol), amides (e.g., dimethylformamide,dimethylacetamide and the like), ethers (e.g., 1,4-dioxane,tetrahydrofuran and the like), ketones (e.g., acetone and the like),glycols(e.g., polyethylene glycol, polypropylene glycol, ethyleneglycol, thiodiglycol, propylene glycol, triethylene glycol, diethyleneglycol and the like), sulfur-containing compounds (e.g., sulfolane,dimethyl sulfoxide and the like), polyfunctional compounds (e.g.,dimethylaminoethanol, diethylaminoethanol, triethanolamine, morpholineand the like), and nitrogen-containing compounds (e.g., 2-pyrrolidone,N-methylpyrrolidone, ε-caprolactam, γ-butyrolactone and the like). Amongthese, glycol ethers and/or polyhydric alcohols are preferable.

The ink composition of the present invention can contain, in addition tothe above ingredients, compounds having the following formulae:

wherein m and n each are an integer of 0-30; and m+n is an integer of0-30,CH₃—(CH₂)_(m)—O—(CH₂CH₂O)_(n)—H  (b)wherein m is an integer of 11-13; and n is an integer of 3-30,

wherein m and n each are an integer of 0-11; m+n is an integer of 9-11;and x is an integer of 3-50, andHO(CH₂CH₂O)_(p)(CH₂CH₂CH₂O)_(q)(CH₂CH₂O)_(r)H  (d)wherein p is an integer of 0-78; q is an integer of 2-15; and r is aninteger of 0-18.

The ink composition of the present invention may include, in addition tothe ingredients mentioned so far, other additives such as amildewproofing agent selected from sodium dehydroacetate, sodiumbenzoate, sodium salt of sorbitan acid and the like, a pH adjustorselected from triethanolamine, sodium hydroxide, sodium nitrate,potassium nitrate and the like, a chelating agent, a rust-proofingagent, an ultraviolet absorber, an acrylic resin, a dye and the like.

Among these, the dye is not particularly limited as long as it is awater-soluble dye, and specific examples of such dyes include an aciddye, a direct dye, a reactive dye and the like. Among these, dyesexcellent in water resistance and light resistance and further in safetycan be used suitably.

Examples of cyan dyes include, for example, C.I. Acid Blues 7, 9, 29,45, 92 and 249, C.I. Direct Blues 1, 2, 6, 15, 22, 25, 71, 76, 79, 86,90, 98, 163, 165, 199 and 202, and C.I. Reactive Blues 1, 2, 7, 14, 15,23, 32, 38, 41, 63, 80 and 95. Among these, at least one cyan dyeselected from C.I. Acid Blue 7, C.I. Acid Blue 9 and C.I. Direct Blue199 is particularly preferable.

Examples of magenta dyes include, for example, C.I. Acid Reds 1, 8, 13,14, 18, 26, 27, 35, 37, 42, 52, 82, 87, 89, 92, 97, 106, 111, 114, 115,134, 186, 249, 254 and 289, C.I. Direct Reds 1, 4, 9, 13, 17, 20, 28,31, 39, 80, 81, 83, 89, 225 and 227, C.I. Direct Oranges 26, 29, 62 and102, and C.I. Reactive Reds 1, 14, 17, 25, 26, 32, 37, 44, 46, 55, 58,60, 66, 74, 79, 96, 97, 141, 147, 180 and 181. Among these, at least onemagenta dye selected from C.I. Acid Red 52, C.I. Acid Red 289, C.I.Reactive Red 58, C.I. Reactive Red 141 and C.I. Reactive Red 180 isparticularly preferable.

Examples of yellow dyes include, for example, C.I. Acid Yellows 1, 7,17, 23, 42, 44, 79 and 142, C.I. Direct Yellows 1, 12, 24, 26, 33, 44,50, 86, 120, 132, 142 and 144, and C.I. Reactive Yellows 1, 5, 11, 13,14, 20, 21, 22, 25, 40, 47, 51, 55, 65 and 67. Among these, at least oneyellow dye selected from C.I. Acid Yellow 17, C.I. Acid Yellow 23 andC.I. Direct Yellow 86 is particularly preferable.

Examples of black dyes include, for example, C.I. Food Black 2, C.I.Direct Blacks 19, 22, 32, 38, 51, 56, 71, 74, 75, 77, 154, 168 and 171,C.I. Reactive Blacks 3, 4, 7, 11, 12 and 17. Among these, at least oneblack dye selected from C.I. Food Black 2 and C.I. Direct Black 154 isparticularly preferable.

The amount of a dye to be added is not particularly limited, but ingeneral, it is in the range of 0.05-20 wt %, and preferably it is in therange of 0.1-15 wt %. The amount of a dye less than 0.05 wt % is notpreferable, because a required color tone and contrast is lessobtainable. Also, the amount of a dye exceeding 20 wt % is notpreferable either, because clogging is more prone to occur in thenozzle.

Also, according to the present invention, there are provided a recordingmethod for recording images comprising allowing ink to adhere to arecording medium and a recording method for recording images comprisingapplying a pressure to eject drops of an ink and to allow the drops toadhere to a recording medium. Both of the methods, by using the inkcomposition of the present invention, not only permit stable ejectionbut also ensure high quality recorded images having excellent waterresistance, excellent excoriation resistance and low-bleeding property.

Furthermore, according to the present invention, there is provided anink head permitting stable ejection and recorded images having highquality, excellent water resistance, excellent excoriation resistanceand low-bleeding property.

More specifically, the ink head of the present invention is constructedto comprise (i) an ink tank retaining an ink composition of presentinvention; (ii) an ink chamber having an ejection orifice through whichdrops are ejected, and having the ink composition fed from the ink tank;and (iii) a piezoelectric element provided inside the ink chamber andcausing a strain in response to a voltage applied to the ink compositionstored in the ink chamber. With the above construction, the presentinvention can provide a piezo-type ink head having high reliability inwhich ink drops are ejected through the ejection orifice according tothe voltage applied to the piezoelectric element.

The ink head of the present invention may comprise, instead of thepiezoelectric element recited in (iii), (iii)′ a heating elementprovided inside the ink chamber for heating the ink composition storedin the ink chamber to create a bubble so that a pressure is applied tothe ink composition. In such a case, the present invention can provide athermal inkjet type ink head in which ink drops are ejected through theejection orifice according to the voltage applied to the heatingelement.

Ink heads of these types can be used for various devices for a recordingpurpose such as an inkjet printer, a Bubble Jet™ printer and the like.

Still further, according to the present invention, there is provided arecorded image which is recorded by using the ink composition of thepresent invention.

To give a more detailed explanation of the present invention, theattached drawings will be referred to.

FIG. 1 is an exploded perspective view showing, in a simplified form,the construction of a piezo-type ink head 1 according to an embodimentof the present invention.

The ink head 1 includes: a head plate 10 having a bottom wall 11 formedof a piezoelectric material and a plurality of ribs 12; a top plate 20provided on the top faces of the ribs 12; and a plurality of ejectionorifices 31, and further includes: a nozzle plate 30 provided on one endof the ribs 12; a rear plate, not illustrated, provided on the other endof the ribs 12; and an ink tank 50 overlying the top plate 20 and havingan opening 51. The plurality of ribs 12 are arranged parallel atintervals of a given length on the bottom wall 11, so that a pluralityof ink chambers 40 are defined by the plurality of ribs 12, the bottomwall 11, the top plate 20, the nozzle plate 30 and the rear plate, notillustrated. The top plate 20 has formed therein a common ink feedpassage 21 that communicates with the ink chambers 40, as well as an inkfeed duct 22 that joins the common ink feed passage 21 and the opening51 of the ink tank 50 together. In the ink tank 50 is retained an inkcomposition 60, which is fed via the common ink feed passage 21 to theink chambers 40.

FIG. 2 is a view showing, in an enlarged form, part of the constructionof the head plate 10, which constitutes the ink head 1 shown in FIG. 1.

Driving electrodes 13 for applying a voltage to the plurality of ribs 12are formed on surfaces, fronting on the ink chambers 40, of the bottomwall 11 of the head plate 10 and of the plurality of ribs 12. Thepiezoelectric material constituting the plurality of ribs 12 ispolarized in the direction of arrow 70, and the plurality of ribs 12function as a piezoelectric element.

The ink head 1 thus constituted is a piezo-type ink head, in which dropsof the ink composition 60 are ejected through the ejection orifices 31according to the voltage applied to the plurality of ribs 12, which arethe piezoelectric element.

There will be explained the principle according to which the inkcomposition 60 is ejected from the ink chambers 40 in the ink head 1.

FIG. 3 is a cross section of the ink head 1 in FIG. 1 viewed along thedirection in which the ink chambers 40 extend. Herein, the operationimplemented for the ink composition 60 to be ejected from an ink chamber40 b will be explained.

When a voltage is not applied to a rib 12 a nor to a rib 12 b, the ribs12 a and 12 b constituting the ink chamber 40 b, that is, when there isno potential difference between a driving electrode 13 b of the inkchamber 40 b and a driving electrode 13 a of a ink chamber 40 a adjacentto the ink chamber 40 b nor between the driving electrode 13 b of theink chamber 40 b and a driving electrode 13 c of the ink chamber 40 cadjacent to the ink chamber 40 b, the ink chamber 40 b is loaded withthe ink composition 60 fed from the ink tank 50 by means of capillaryaction. Likewise, the ink chambers 40 a and 40 c are also loaded withthe ink composition 60.

When a voltage is applied to the driving electrode 13 a and drivingelectrode 13 c, a potential difference is caused between the drivingelectrode 13 b and driving electrode 13 a and between the drivingelectrode 13 b and driving electrode 13 c to apply the voltage to theribs 12 a and 12 b, which constitute the ink chamber 40 b. The voltageinduces electric fields within the ribs 12 a and 12 b in the directionsof arrows 71 and 72, and the actions of the electric fields causestrains on the ribs 12 a and 12 b, which constitute the ink chamber 40b, with the result that the ribs 12 a and 12 b each are deformed into ashape having a prominence on the ink chamber 40 b side. This gives riseto a pressure wave to apply a high pressure to the ink composition 60loaded in the ink chamber 40 b, whereby drops of the ink composition 60are ejected through the ejection orifices 31.

When the application of the voltage to the driving electrode 13 a anddriving electrode 13 c is stopped, the ribs 12 a and 12 b recover totheir original shape, and the ink chamber 40 b recovers to its originalvolume. In turn, an amount of the ink composition 60 to compensate foran increase in volume given by the restoration is fed from the ink tank50 via the common ink feed passage 21, so that the ink chamber 40 breturns to the initial state in which it is loaded with the inkcomposition 60.

In the ink head 1 of the present embodiment, as described above, the inktank 50 retains the ink composition 60 of the present embodiment, andthe ink composition 60 is fed to the ink chambers 40 and ejected asdrops through the ejection orifices 31. Thus, the ink head 1 permitsstable ejection of the drops of the ink composition 60 through theejection orifices 31. By using such an ink head, it is possible torealize a piezo-type inkjet recording device having high reliability.

According to the present embodiment, the ribs 12, which constitute theink chambers 40, are formed of the piezoelectric material, and serves asthe piezoelectric element. However, the material of the ribs 12 is notlimited thereto, but the ribs forming the ink chambers may be formed ofa material other than the piezoelectric material, and the piezoelectricelement may be provided inside or outside the ribs.

FIG. 4 is an exploded perspective view showing, in a simplified form,the construction of an ink head 2 according to another embodiment of thepresent invention. FIG. 5 is a plan view showing part of theconstruction of the ink head 2 shown in FIG. 4. However, in FIG. 5, thetop plate 20 and the ink tank 50 in FIG. 4 are omitted to avoidcomplexity of the illustration and to not make understanding difficult.The ink head 2 of the present embodiment is similar to the ink head 1,and corresponding components are denoted by the same reference numeraland explanations thereof are omitted.

It is to be noted that a head plate 100 comprises: a substrate 101; aplurality of ribs 102 arranged parallel at intervals of a given lengthon the substrate 101; a heater 103 provided as a heating element on thesurface of the substrate 101 fronting on the ink chambers 40; anddriving electrodes 104 and 105 for applying a voltage to the heater 103.

The ink head 2 thus constituted is a thermal inkjet type ink head inwhich drops of the ink composition 60 are ejected through the ejectionorifices 31 according to a voltage applied to the heater 103 serving asthe heating element.

There will be explained the principle according to which the inkcomposition 60 is ejected from the ink chambers 40 in the ink head 2.

FIG. 6 is a view showing the state in which a drop 61 of the inkcomposition 60 is ejected from the ink chamber 40.

In FIG. 6, the ink head 2 is shown in cross section taken along line I-Iof FIG. 5.

When a voltage is not applied to the heater 103, as in the case where inthe ink head 1, the voltage is not applied to the ribs 12, the inkchamber 40 is loaded with the ink composition 60.

When a voltage is applied to the heater 103 by means of the drivingelectrodes 104 and 105, the heater 103 generates heat, so that the inkcomposition 60 loaded in the ink chamber 40 is heated to create a bubble62. This gives rise to a pressure wave to apply a high pressure to theink composition 60 loaded in the ink chamber 40, whereby the drop 61 ofthe ink composition 60 is ejected through the ejection orifice 31.

When the application of the voltage to the heater 103 is stopped, theink composition 60 loaded in the ink chamber 40 is cooled, the bubble 62eliminates, and an amount of the ink composition 60 to compensate for anincrease in volume given by the restoration is fed from the ink tank 50via the common ink feed passage 21, so that the ink chamber 40 returnsto the initial state in which it is loaded with the ink composition 60.

In the ink head 2 of the present embodiment, like the ink head 1, theink tank 50 retains the ink composition 60, which is fed to the inkchambers 40 and ejected as the drops 61 through the ejection orifices31. Thus, the ink head 1 permits stable ejection of the drops 61 of theink composition 60 through the ejection orifices 31. By using such anink head, it is possible to realize a thermal inkjet type recordingdevice having high reliability.

EXAMPLES

Next, Examples directed to the ink composition of the present inventionand Comparative Examples are shown below.

Examples and Comparative Examples shown in Table 1 were constituted.Herein, in all the cases, ingredients were shown in parts by weight tomake the total of 100.

TABLE 1 Examples Comparative Examples 1 2 3 4 5 6 1 2 3 4 5 Compound offormula (1) 1 2 1.5 2 1.5 2.5 1 Organic 1,2-Hexanediol 3 6 7 7 7 7 3 6 99 10 solvents Glycerol 5 5 3 5 5 3 20 2-Pyrrolidone 5 2 2 5 5 5 5 2 2 515 Isopropanol 1 1 1 1 5 Trimethylolpropane 5 5 2 Pigments Pigment Blue15:3 2.5 4 Pigment Blue 15:4 2 Pigment Red 122 4 Pigment Yellow 74 3.5Pigment Yellow 180 3.5 Carbon black 5 5 Pigment Blue 17 2 Pigment Red 583 Pigment Yellow 13 2 Others Formula (a) (m + n = 10) 1 1 Formula (b) (m= 12, n = 15) 0.5 0.5 1.5 Diisobutylsulfosuccinic acid 1 Dodecyl sulfate1.5 Acrylic resin 1 4 3 4 4 3 10 3 5 2 2 Ammonium sulfate 0.5 0.7 WaterBl Bl Bl Bl Bl Bl Bl Bl Bl Bl Bl (Conductivity μS/cm) (250) (10) (1)(0.5) (4.0) (3.0) (290) (200) (10) (1) (80) Bl: Balance(1) Conductivity

The conductivities at 25° C. of the ink compositions with ingredientsshown in Table 1 were measured using CM-40G manufactured by TOA Corp.

(2) High Speed Printing

The printable number of sheets in continuous printing at 7 PPM using theabove composition used at a printing density of 5% by a modified printerAJ2000 manufactured by Sharp Corporation was determined. The printablenumber of sheets during the period after the ink tank was loaded withink until it became empty was about 200.

In the experiment, each ink composition was reloaded at the time whenthe ink tank became empty, the recharging operation was repeated untilbeing not able to print, and the number of printed sheets obtained bythat time was decided as the printable number of sheets. The mark ◯ wasput when the printable number of sheets exceeded 200, the mark Δ was putwhen the printable number of sheets was 150-200, and the mark × was putwhen the printable number of sheets was 150 or smaller.

(3) Velocity of Drying

Printing was performed in the same manner as in (2), and 15 secondsdirectly after the completion of the printing, the same type of sheet asused in the printing was applied onto the printing surface of the mediumunder predetermined conditions, and the mark × was put when the ink wastransferred and the mark ◯ was put when the ink was not transferred.

(4) Evaluation on Bleeding

A particular pattern was printed on a PPC sheet (SF4AM3 manufactured bySharp Corporation) in the same manner as in (2) and was allowed to standfor one day to make an evaluation using a relative value when the valueof a line width of the pattern free of bleeding is assumed to be 100.

The mark ◯ was put when the value was 150 or smaller, the mark Δ was putwhen the value was in the range of 150 to 250, and the mark × was putwhen the value was greater than 250.

(5) Evaluation on Foaming

The ink compositions were shaken for 20 seconds and the heights of theirfoams after five minutes were measured. The mark ◯ was put when theheight was 10 mm or lower and the mark × was put when it exceeded 10 mm.

Table 2 shows the results.

TABLE 2 Examples Comparative Examples 1 2 3 4 5 6 1 2 3 4 5 Conductivity0.25 0.1 0.4 0.6 0.6 7.0 0.8 0.2 0.1 0.3 8.1 (mS/cm) High speed ◯ ◯ ◯ ◯◯ ◯ Δ X Δ ◯ X printing Velocity ◯ ◯ ◯ ◯ ◯ ◯ X ◯ X X ◯ of drying Bleeding◯ ◯ ◯ ◯ ◯ ◯ Δ ◯ ◯ X ◯ Foaming ◯ ◯ ◯ ◯ ◯ ◯ X ◯ X X X

Next, inks of Examples 1-5 and Comparative Examples 1-3 were combined asshown in FIG. 3 and the resulting three-color blacks each composed ofcyan, magenta and yellow in the proportion of 1:1:1 were printed onglossy paper.

TABLE 3 Cyan Magenta Yellow Combination 1 Pigment Blue Pigment RedPigment Yellow 15:3 122 74 Combination 2 Pigment Blue Pigment RedPigment Yellow 15:3 122 180 Combination 3 Pigment Blue Pigment RedPigment Yellow 15:4 122 74 Combination 4 Pigment Blue Pigment RedPigment Yellow 15:4 122 180 Combination 5 Pigment Blue Pigment RedPigment Yellow 17 58 13

A single color black prepared from an ink of Example 6 and thethree-color blacks prepared from the inks of Examples and ComparativeExamples were subjected to a calorimeter (X-Rite 938 manufactured byMacbeth Co.). Herein, the differences of the values of the ink ofExample 6 from the values of each three-color black were evaluated oncriteria a*, b*. When a value expressed by the equationΔa*b*={(A1−A2)²+(B1−B2)²}^(1/2), in which in Example 6, a*=A1 and b*=B1,whereas in each three-color black, a*=A2 and b*=B2, the mark ◯ was putwhen it was Δa*b*≦20, whereas the mark × was put when it was Δa*b*>20.The results are shown in Table 4.

TABLE 4 Combination 1 2 3 4 5 Result ◯ ◯ ◯ ◯ X

From this, it was found that a particular combination is suitable forpigment inks.

As described hereinabove, the ink composition of the present inventionpermits ink ejection stability even in high speed printing and has anexcellent property of quick drying of ink and low-bleeding property on amedium, thereby achieving high quality print.

1. An ink composition comprising: (a) an aqueous medium having aconductivity of 250 μS/cm (at 25° C.) or lower; (b) a pigment; and (c) acompound represented by the formula (I):

wherein m and n each are an integer of 0-11; m+n is an integer of 9-11;x is an integer of 5-9; and y is a number between 2.5-5.
 2. The inkcomposition of claim 1, wherein the mixed compound of the formula (I) iscontained in a critical micelle concentration or above.
 3. The inkcomposition of claim 1, wherein its conductivity is 8 mS/cm (at 25° C.)or lower.
 4. The ink composition of claim 1, further comprising 10-70 wt% of a water-soluble organic solvent.
 5. The ink composition of claim 4,wherein the water-soluble organic solvent comprises at least one organicsolvent having a vapor pressure higher than that of water.
 6. The inkcomposition of claim 4, wherein the water-soluble organic solventcomprises at least one organic solvent having a vapor pressure of 0.05mmHg or lower at 20° C.
 7. The ink composition of claim 4, wherein thewater-soluble organic solvent is selected from glycol ethers and/orpolyhydric alcohols.
 8. The ink composition of claim 1, wherein thepigment is a self-dispersion type pigment in which a carboxylic acid ora sulfonic acid is introduced on its surface.
 9. The ink composition ofclaim 1, wherein the pigment is C.I. Pigment Blue 15:3 or C.I. PigmentBlue 15:4.
 10. The ink composition of claim 1, wherein the pigmentcomprises at least one pigment selected from the group consisting ofC.I. Pigment Red 122, C.I. Pigment Red 209 and C.I. Pigment Violet 19.11. The ink composition of claim 1, wherein the pigment comprises atleast one pigment selected from the group consisting of C.I. PigmentYellow 74, C.I. Pigment Yellows 128 and 138, and C.I. Pigment Yellow180.
 12. The ink composition of claim 1, wherein the pigment is a carbonblack.
 13. An ink set comprising: an ink composition of claim 1, whereinthe pigment is C.I. Pigment Blue 15:3 or C.I. Pigment Blue 15:4; an inkcomposition of claim 1, wherein the pigment comprises at least onepigment selected from the group consisting of C.I. Pigment Red 122, C.I.Pigment Red 209 and C.I. Pigment Violet 19; and an ink composition ofclaim 1, wherein the pigment comprises at least one pigment selectedfrom the group consisting of C.I. Pigment Yellow 74, C.I. PigmentYellows 128 and 138, and C.I. Pigment Yellow
 180. 14. An ink setcomprising: an ink composition of claim 1, wherein the pigment is C.I.Pigment Blue 15:3 or C.I. Pigment Blue 15:4; an ink composition of claim1, wherein the pigment comprises at least one pigment selected from thegroup consisting of C.I. Pigment Red 122, C.I. Pigment Red 209 and C.I.Pigment Violet 19; an ink composition of claim 1, wherein the pigmentcomprises at least one pigment selected from the group consisting ofC.I. Pigment Yellow 74, C.I. Pigment Yellows 128 and 138, and C.I.Pigment Yellow 180; and an ink composition of claim 1, wherein thepigment is a carbon black.
 15. A recording method for recording imagescomprising allowing an ink composition of claim 1 to adhere to arecording medium.
 16. A recording method for recording images comprisingapplying a pressure to eject drops of an ink and to allow the drops toadhere to a recording medium, using an ink composition of claim 1 as theink.
 17. An ink head comprising: (i) an ink tank retaining an inkcomposition of claim 1; (ii) an ink chamber having an ejection orificethrough which drops are ejected, and having the ink composition fed fromthe ink tank; and (iii) a piezoelectric element provided inside the inkchamber and causing a strain in response to a voltage applied to the inkcomposition stored in the ink chamber.
 18. An ink head comprising: (i)an ink tank retaining an ink composition of claim 1; (ii) an ink chamberhaving an ejection orifice through which drops are ejected, and havingthe ink composition fed from the ink tank; and (iii) a heating elementprovided inside the ink chamber for heating the ink composition storedin the ink chamber to create a bubble so that a pressure is applied tothe ink composition.
 19. A recorded image which is recorded by using anink composition of claim 1.